Promethium is the most elusive member of the lanthanide family, a row of 15 metals marooned in the periodic table’s southern territories. Discovered in 1945, the element was named after the Titan Prometheus, who stole fire from the gods in Greek mythology. Researchers estimate that less than 1 kilogram of it currently exists naturally in the Earth’s crust, and its radiation has previously been harnessed to power pacemakers and spacecraft. Despite its importance, Pm has been conspicuously absent from the experimental studies of lanthanides, impeding our full comprehension of the so-called lanthanide contraction phenomenon, a fundamental aspect of the periodic table quoted in general chemistry textbooks. My present work involves the design and synthesis of diglycolamide (DGA) ligand to chelate radioactive promethium (¹⁴⁷Pm radionuclide with a half-life of 2.62 years), making a stable chemical ‘complex’ — a compound in which three surrounding molecules girdle central Pm³⁺ ion. This feat of synthesis enabled the team to study how the element bonds with other atoms in a water solution. Due to the limited availability of radioisotope ¹⁴⁷Pm, a surrogate crystal of [Sm(DGA)₃][Sm(NO₃)₆].3C₂H₅OH was developed. The homoleptic Pm³⁺ complex in an aqueous solution was studied using synchrotron X-ray absorption spectroscopy and quantum chemical calculations to establish promethium's coordination structure and bond distances. These fundamental insights allow a complete structural investigation of a full set of isostructural lanthanide complexes, ultimately capturing the lanthanide contraction in solution solely based on experimental observations. Our results showed accelerated shortening of bonds at the beginning of the lanthanide series, which can be correlated to the separation trends shown by diglycolamides. Lately, we have also been focusing on synthesizing redox-active DGA ligands, which could lead to the development of new environmentally friendly and efficient separation of trivalent lanthanides.

Ilja Popovs

Oak Ridge National Laboratory Physical Sciences Directorate

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